The present embodiments relate to wireless communications systems and, more particularly, to improved handover for Digital Video Broadcast-Handheld (DVB-H) for a wireless communication system.
Wireless communications are prevalent in business, personal, and other applications, and as a result the technology for such communications continues to advance in various areas. One such advancement includes the use of spread spectrum communications, including that of code division multiple access (CDMA) which includes wideband code division multiple access (WCDMA) cellular communications. In CDMA communications, user equipment (UE) (e.g., a hand held cellular phone, personal digital assistant, or other) communicates with a base station, where typically the base station corresponds to a “cell.” CDMA communications are by way of transmitting symbols from a transmitter to a receiver, and the symbols are modulated using a spreading code which consists of a series of binary pulses. The code runs at a higher rate than the symbol rate and determines the actual transmission bandwidth. In the current industry, each piece of CDMA signal transmitted according to this code is said to be a “chip,” where each chip corresponds to an element in the CDMA code. Thus, the chip frequency defines the rate of the CDMA code. WCDMA includes alternative methods of data transfer, one being frequency division duplex (FDD) and another being time division duplex (TDD, where the uplink and downlink channels are asymmetric for FDD and symmetric for TDD. Another wireless standard involves time division multiple access (TDMA) apparatus, which also communicate symbols and are used by way of example in cellular systems. TDMA communications are transmitted as a group of packets in a time period, where the time period is divided into time slots so that multiple receivers may each access meaningful information during a different part of that time period. In other words, in a group of TDMA receivers, each receiver is designated a time slot in the time period, and that time slot repeats for each group of successive packets transmitted to the receiver. Accordingly, each receiver is able to identify the information intended for it by synchronizing to the group of packets and then deciphering the time slot corresponding to the given receiver. Given the preceding, CDMA transmissions are receiver-distinguished in response to codes, while TDMA transmissions are receiver-distinguished in response to time slots.
New standards for Digital Video Broadcast (DVB) standards are currently being developed to permit streaming video reception by portable user equipment. DVB packets or data streams are transmitted by Orthogonal Frequency Division Multiplex (OFDM) transmission with time slicing. With OFDM, multiple symbols are transmitted on multiple carriers that are spaced apart to provide orthogonality. An OFDM modulator typically takes data symbols into a serial-to-parallel converter, and the output of the serial-to-parallel converter is considered as frequency domain data symbols. The frequency domain tones at either edge of the band may be set to zero and are called guard tones. These guard tones allow the OFDM signal to fit into an appropriate spectral mask. Some of the frequency domain tones are set to values which will be known at the receiver, and these tones are termed pilot tones or symbols. These pilot symbols can be useful for channel estimation at the receiver. An inverse fast Fourier transform (IFFT) converts the frequency domain data symbols into a time domain waveform. The IFFT structure allows the frequency tones to be orthogonal. A cyclic prefix is formed by copying the tail samples from the time domain waveform and appending them to the front of the waveform. The time domain waveform with cyclic prefix is termed an OFDM symbol, and this OFDM symbol may be upconverted to an RF frequency and transmitted. An OFDM receiver may recover the timing and carrier frequency and then process the received samples through a fast Fourier transform (FFT). The cyclic prefix may be discarded and after the FFT, frequency domain information is recovered. The pilot symbols may be recovered to aid in channel estimation so that the data sent on the frequency tones can be recovered. A parallel-to-serial converter is applied, and the data is sent to the channel decoder.
Referring to FIG. 1, rectangles 100 and 102 represent DVB packets of a current data stream 104. The time between the start of DVB packets 100 and 102 is the delta-t time. Time between the DVB packets 100 and 102 is off time. The delta-t time is transmitted with other header information in each DVB packet to inform the DVB-H receiver when the next packet will arrive. The delta-t time is relative rather than absolute, so the DVB-H clock only needs to accurately measure the time from one packet to the next packet. Moreover, if a packet is lost, the DVB-H receiver may continue to monitor the carrier frequency 104 until the next packet arrives. This form of time slicing advantageously permits the DVB-H receiver to enter a low power mode or sleep mode after packet 100 is received. The DVB-H receiver subsequently wakes up in response to a timed interrupt to receive the next data packet 102. This method of operation greatly reduces power consumption by the DVB-H receiver and prolongs battery life. Alternatively, the DVB-H receiver may use this time between packets to monitor alternative carrier frequencies of nearby cells. These alternative carrier frequencies are provided in a Network Information Table (NIT) for each network. The NIT includes an NIT-actual, having a list of frequencies for the current network, and several NIT-other lists, each having a list of frequencies for an adjacent network.
Referring now to FIG. 2, there is an exemplary DVB multi-frequency network (MFN). The MFN includes three cells 200, 202, and 204 operating at frequencies f1, f2, and f3, respectively. Cell 200 has a maximum radius d=3.2 km, representing approximately 0 dB gain for 16 QAM at ⅔ code rate. Radii d/2 and 2d represent 10 dB and −10 dB gain, respectively. For transmitter power of 5 kW, a digital video broadcast handheld (DVB-H) receiver 210 moving at 120 km/h would receive seamless quality of service (QoS) if a handover is completed in 48 seconds or less. This represents d/2 or 1.6 km. There are several problems, however, that are somewhat unique to DVB handovers.
FIG. 3A illustrates two data streams from neighboring cells. The upper data stream includes a series of OFDM packets N through N+5. The lower data stream includes a series of OFDM packets N+3 through N+7. The upper data stream is transmitted from a current DVB transmitter 200 operating at frequency f1 which is not in synchronization with a neighboring DVB transmitter in another cell 204 operating at frequency f3. A first packet N on frequency f1 is followed in time by packet N+3 on frequency f3. A second packet N+1 on frequency f1 is followed in time by packet N+4 on frequency f3. This problem typically occurs due to differing internet backbone delays and precludes a simple handover from cell 200 to cell 204.
FIG. 3B illustrates another problem when multiple data streams are transmitted with significant time shifts. Here, packets N, N+1, and N+2 of a first data stream and packets M, M+1, and M+2 of a second data stream are transmitted on frequency f1 from DVB transmitter 200. The same data streams are transmitted on frequency f3 from DVB transmitter 204, but packets M+1, M+2, and M+3 are shifted to the left in time. FIG. 3C illustrates yet another problem of time shifting with multiple data streams. Here, packets N, N+1, and N+2 of a first data stream and packets M, M+1, and M+2 of a second data stream are transmitted on frequency f1 from DVB transmitter 200 as in FIG. 3B. However, packets N+4, N+5, and N+6 of the first data stream and packets M+4, M+5, and M+6 of the second data stream are transmitted on frequency f3 from DVB transmitter 204 with a significant time shift. These problems significantly complicate handovers as the DVB-H moves from cell to cell in the MFN.